The present invention is in the field of photochromic glasses and relates to refractive index-corrected photochromic glasses useful in the manufacture of photochromic ophthalmic lenses.
Photochromic glasses comprising crystallites of a silver halide as the phototropic phase have been described by Armistead and Stookey in U.S. Pat. No. 3,208,860. Eppler and Stookey disclose, in U.S. Pat. No. 3,197,296, a family of refractive-index-corrected silver halide photochromic glasses suitable for use in ophthalmic lenses.
Silver-free photochromic glasses exhibiting phototropic behavior analogous to that of the silver halide photochromic glasses, but comprising copper-cadmium halides in the phototropic phase, are reported by Araujo in U.S. Pat. No. 3,325,299. Additional copper-cadmium halide photochromic glasses are disclosed in U.S. Pat. No. 3,954,485 to Seward and Tick, a patent dealing with the manufacture of silver-free polarizing photochromic glasses.
The above-described silver-free photochromic glasses, hereinafter referred to as copper-cadmium halide photochromic glasses, offer certain economic and functional advantages over silver halide photochromic glasses. Included among these advantages are a smaller batch cost, due to the elimination of silver, a reduced dependence of photochromic properties on glass temperature, and a more desirable relationship between the intensity of the incident light and the photochromic darkening induced thereby in the glass.
Glasses which are to be utilized as ophthalmic lens glasses must be refractive index-corrected, i.e., they must exhibit a refractive index (n.sub.D) in the range of about 1.52-1.54, preferably about 1.523, in order to be compatible with presently-available ophthalmic lens grinding and testing equipment. It is also essential that the glass be of optical quality, i.e., low in haze and free of inclusions and the like.
In the case of photochromic glasses for ophthalmic use, photochromic performance which is at least approximately equivalent to present commercially-available photochromic ophthalmic lenses is also required. Important elements of photochromic performance include rapid photochromic darkening upon exposure to actinic radiation, rapid fading in the absence thereof, and the attainment of an adequate degree of darkening in ophthalmic thicknesses (about 2 mm) when exposed to sunlight.
Refractive index-corrected copper-cadmium halide photochromic glasses of optical quality would find ready application in the manufacture of photochromic ophthalmic lenses, provided that acceptable photochromic properties were exhibited thereby. However, prior art refractive index-corrected base glasses used in the silver halide photochromic system, i.e., the base glasses described in the aforementioned U.S. Pat. No. 3,197,296 to Eppler et al., do not generally provide a good base for copper-cadmium halide photochromic phases. Substitution of copper-cadmium halide for silver halide in these glasses typically results in a product exhibiting relatively poor photochromic performance.
It is a principal object of the present invention to provide refractive index-corrected glass compositions suitable for making copper-cadmium halide photochromic glass lenses of optical quality, and ophthalmic lenses provided therefrom, which exhibit excellent photochromic properties.
It is a further object of provide glass compositions of this type which can be easily melted and formed utilizing existing glass-working techniques, yet which provide ophthalmic products which can be chemically strengthened by known methods and which meet or exceed existing standards for chemical durability and weatherability.
Other objects and advantages of the invention will become apparent from the following description thereof.